Traditionally, cellular handset radio receivers are designed to receive a signal modulated to a single carrier frequency. For example, the radio receiver may comprise a direct-conversion receiver, where an analog and digital baseband signal processing circuit downconverts the input signal to I and Q baseband signals using two local oscillator (LO) signals, fLO,I and fLO,Q, respectively. Both fLO,I and fLO,Q have the same frequency, which is equal to the carrier frequency of the input signal, and a 90° phase difference, which prevents loss of the received information during processing. The analog and digital baseband signal processing circuit further processes the downconverted I and Q signals with analog and digital baseband circuits to retrieve the wanted signal.
If the wanted signal contains several adjacent frequency channels, they can in principle be processed with a single radio receiver having a single front-end amplifier connected to I and Q downconversion mixers followed by a single analog and digital baseband I/Q signal processing circuit as long as the total bandwidth of the wanted signal does not exceed the bandwidth of the receiver. For example, if the wanted signal contains N frequency channels, and the bandwidth of one channel is fBW, the total bandwidth of the wanted signal may be represented by NfBW. To minimize the bandwidth used in baseband/IF signal processing, and therefore to minimize power dissipation of the wireless receiver, the LO signal used to generate fLO,I and fLO,Q should be placed in the middle of the received signal band. The bandwidth of the downconverted wanted signal then becomes NfBW/2. If the wanted signal contains an even number of channels, each having the same bandwidth, the LO signal can be placed between two adjacent channels and all channels are processed as in a low-IF receiver. If the wanted signal contains an odd number of received adjacent channels, each having the same bandwidth, the LO signal should have a frequency equal to the center frequency of one of the channels, where this channel would be processed like in a direct-conversion receiver and the other channels would be processed as in a low-IF counterpart receiver.
Carrier aggregation refers to the simultaneous wireless reception of several signal channels associated with different frequencies in Long Term Evolution (LTE) wireless systems. A similar situation existing in Global System for Mobile communications (GSM) and High-Speed Downlink Packet Access (HSDPA) systems is generally referred to using the terms dual carrier or multi-carrier. While the term “carrier aggregation” is generally used herein, it will be appreciated that the following also applies to dual-carrier and multi-carrier systems.
Inter-band carrier aggregation refers to carrier aggregation where the wanted signal channels are in different reception bands. In practice, an off-chip passive radio frequency (RF) bandpass filter is used before the receiver integrated circuit (IC) to attenuate potential out-of-band blocking signals to levels that the receiver IC can tolerate. For inter-band carrier aggregation, a separate off-chip RF filter is needed for each reception band, where each filter is usually followed by a dedicated low-noise amplifier (LNA) or LNA input stage tuned to that reception band. The bandwidth of the LNA following the off-chip filter may be insufficient for simultaneous reception of channels at different reception bands. A separate LO signal having a different frequency is therefore needed for each reception band for signal downconversion. Each separate LO signal (I/Q) requires a separate analog and digital baseband/IF signal processing circuit. Inter-band carrier aggregation therefore requires parallel radio receivers, e.g., one receiver chain for each simultaneously utilized reception band.
Intra-band carrier aggregation refers to carrier aggregation where all wanted signal channels are within a single reception band, e.g., the passband of one off-chip RF filter. In contiguous intra-band carrier aggregation, there are at least two wanted signal channels and all wanted signal channels are adjacent or next to each other. In non-contiguous intra-band carrier aggregation, all wanted signal channels are not adjacent, e.g., there may be space in the frequency domain between some of the signal channels. There may also be blocking signals between wanted signal channels.
In intra-band carrier aggregation, a single off-chip RF filter and one LNA are generally sufficient because all wanted channels are within the passbands of the filter and LNA, e.g., two or more channels can be received using one RF IC input (single-ended or balanced). For example, assume fBW,L represents the RF bandwidth of the wanted signal channel at the lowest carrier frequency of fC,L, and fBW,H represents the RF bandwidth of the wanted signal channel at the highest carrier frequency of fC,H. The total bandwidth of the wanted signal may be represented by:fBW,TOT=fC,H−fC,L+(fBW,H+fBW,L)/2.  (1)It will be appreciated that there may be “empty” channels between the two mentioned channels. If fBW,TOT/2 is less than or equal to the maximum available bandwidth of the baseband/IF analog and digital signal processing circuits of the receiver, the contiguous and non-contiguous intra-band carrier aggregation may be implemented with a single radio receiver utilizing only one LO signal. The different wanted channels are separated/detected in practice in the digital back-end.
If, however, fBW,TOT/2 exceeds the maximum available bandwidth of the baseband/IF analog and digital signal processing circuits, a single receiver chain is not sufficient for contiguous or non-contiguous intra-band carrier aggregation. Further, if blocking signals, which can exist in the frequency bands between the non-contiguous wanted signal channels, have power levels that cannot be tolerated in the analog and/or digital signal processing circuits of the receiver, a single receiver chain is not sufficient for non-contiguous intra-band carrier aggregation. In these cases, the signal processing must be divided in the frequency domain into two or more parallel chains, which requires that the signal be downconverted in parallel I/Q mixers using at least two LO signals with different frequencies. As a result, the received signal must be divided into two or more parallel chains before being applied to the downconversion mixers.
A straightforward option is to use two parallel receiver ICs and connect them to the same RF input. Another option is to use two separate receivers on the same IC and connect them in parallel to the same RF input. Both examples require two parallel LNAs. Because LNAs typically use on-chip inductors, which require large silicon area, using two or more parallel LNAs requires a significant amount of silicon area. In addition, the RF input of the receiver IC has to be matched with sufficient accuracy to a specific impedance level, usually 50Ω, because the LNA input impedance affects the frequency response of the preceding off-chip RF filter. The LNA input has to be matched in all modes of operation, including when there is only one active LNA or when there are two or more parallel LNAs. In addition, the noise figure (NF) of all active LNAs must meet the same requirements, which sets the requirements for the minimum size and bias current of amplifying transistors of the LNAs. The requirements for sufficient input matching and NF regardless of the number of parallel LNAs means that the number of parallel devices connected to the RF input increases relative to the case where only one LNA is used in all modes of receiver operation. The higher number of parallel input devices means higher parasitic capacitances at the receiver RF input, which causes problems with input matching and/or the need for additional off-chip matching components. Using multiple parallel LNAs also significantly increases the supply current of the RF front-end. Thus, the use of parallel LNAs in parallel receivers is not desirable, especially if they use off- or on-chip inductors.
Another solution uses a single LNA capable of amplifying all wanted input signal channels in intra-band carrier aggregation, and then dividing the signal chain into two or more parallel signal processing chains. In the following it is assumed that the LNA is followed by a passive current-mode I/O downconversion mixer, and the resistive input impedance of the mixer forms part of the LNA load impedance. The division of one signal chain into two parallel chains may be implemented by connecting the inputs of two passive current-mode I/Q mixers to the LNA output. The use of parallel passive current-mode I/Q mixers maintains high linearity because highly linear circuit blocks are placed in parallel. However, when multiple passive I/Q mixers are connected to the LNA output, the LNA load impedance decreases relative to the case where only one I/Q mixer is connected to the LNA output. The LNA may use feedback from the node where the parallel I/Q mixers are connected. For example, when the LNA comprises a resistive-feedback LNA, the LNA performance parameters (like gain, input matching, and NF) may deteriorate. When another passive current-mode I/Q mixer is connected to the LNA output, the LNA output signal current is divided between the two parallel passive I/Q mixers, which means that the signal gain (or effective transconductance) provided by the RF front-end decreases, which leads to higher receiver NF. One way to address this problem is to increase the equivalent transconductance in the LNA, which in practice increases the LNA power consumption and/or the parasitic capacitances in the LNA. Unfortunately, larger parasitic capacitances may deteriorate the LNA input matching and NF, and lower the maximum frequency of operation. Moreover, the higher power consumption of such a solution is undesirable in portable devices.
Thus, alternate solutions are needed in the RF front-end for enabling intra-band carrier aggregation when processing the received signal requires using two or more separate LO signals with different frequencies.